The present invention relates to a process for the conversion into a usable condition of actinide ions contained in the solid residue of a sulfate reprocessing process for organic, actinide-containing radioactive solid waste, which actinide ions together with cationic impurities are present in the solid residue in the form of water soluble sulfato complexes.
Among sulfate reprocessing processes for organic solid wastes are included such decomposition processes, as those by which the wastes are oxidatively decomposed in a sulfate- or bisulfate-melt at high temperatures, for example, at 800.degree. C., as well as those by which the wastes are treated with comparatively highly concentrated sulfuric acid, for example, with 90%, or with concentrates H.sub.2 SO.sub.4 (so-called wet incineration process).
Wet incineration processes are processes for the treatment of combustible, solid waste materials with sulfuric acid at raised temperature, by which simultaneously and/or afterwards the waste material exposed by the reaction is brought into contact with nitric acid or nitrogen dioxide. By this means, carbon-containing material is oxidized to non-combustible gas products and to a non-combustible residue having low volume. In the field of nuclear technology, especially in the field of the th reprocessing of exposed nuclear fuel- and/or fertile materials, such combustible solid wastes, which can be comprised, for example, of spent ion-exchange resin, paper, rubber gloves, synthetic parts of different materials, etc., contain radioactive materials, especially actinides, as, for example, uranium and plutonium. With regard to doing as little damage to the environment as possible by radioactive material, it is necessary that radioactive wastes of whatever type undergo as extensive a volume reduction as possible before solidification in a long term stable matrix and consequent final storage. Here it is desirable not only for an improved waste treatment but also for the reuse of certain valuable materials, such as, for example, actinides, to remove the actinides present in the waste, especially uranium and plutonium, before the solidification of the wastes.
One of the process methods useful for this is wet combustion, as described in a process variation example in German Published Patent Application No. 23 47 631, corresponding generally to U.S. Pat. No. 3,957,676. Here, the actinide compounds contained in the solid waste are converted by the hot sulfuric acid into sulfato complexes, which, because of their low solubility in the relatively concentrated sulfuric acid, precipitate and form a solid reaction residue with other constituents of the waste.
For subsequent treatment of solid residues containing actinide ions and sulfate ions of sulfate reprocessing processes for organic solid wastes, it is known for example to leach and wash this solid residue with diluted sulfuric acid, to treat the combined leaching and washing solutions with an organic extraction agent solution to transfer the actinides from the aqueous phase into the organic phase, to treat the organic phase charged with actinides with nitric acid for the re-extraction of actinides into the aqueous phase, thereafter to precipitate the actinides from the aqueous solution with, for example, oxalic acid, and to calcinate the precipitated oxalates or to dissolve them in HNO.sub.3. See, Radioactive Waste Management, Volume 2, Int. Atomic Energy Agency, Vienna, 1984, No. IAEA-CN-43/44. pages 335 to 346.
In still another method for the subsequent treatment of the residue containing actinides with or without sulfate ions which residue was absorbed with water to form an aqueous solution, a process for the selective separation of plutonium and uranium can also be used, by which sulfuric acid is added to the aqueous solution containing Pu.sup.4+ and UO.sub.2.sup.2+ in amounts such that the uranium forms anionic sulfato complexes, then a cationic surfactant is added to the solution to form a uranium containing precipitate, the resulting precipitate then is separated from its mother liquor in a known way; after separation of the uranium-containing precipitate, the pH value of remaining mother liquor is changed so that henceforth the Pu.sup.4+ forms anionic sulfato complexes, then a cationic surfactant is added to form a plutonium containing precipitate, the resulting precipitate then is separated from its mother liquor in a known way, and the plutonium is recovered from the precipitate by means of calcination. See German Published Patent Application No. 32 24 803.
For all known processes, the final product is actinide dioxide. Depending on the separation method used, the actinide dioxide could contain residual amounts of sulfate ions or, if it was calcined at high temperatures, could be insoluble. But both of these disadvantages reduce the reuseability of the product of the process.
Moreover, with the use of this process the plutonium loss can reach, for example, up to 4 or 5 weight %, respectively. Furthermore, the known processes are relatively expensive regarding the handling and number of process steps.